Nucleosomes from Drosophila embryo and rat liver nuclei have been fractionated on the basis of differences in in situ internucleosome packing interactions by elution from micrococcal nuclease digested nuclei with increasing concentrations of NaCl. The subfractions differ in content of nonhistone proteins and of variants of histone H1 and H3. They also differ in stoichiometry of associated H1 and in susceptibility to nuclease digestion. The DNA sequences associated with the fractions will be investigated by DNA . DNA and DNA . RNA hybridization studies to determine the functional significance of this fractionation. The role of non-histone proteins and the histone H1 variants in mediating nucleosome interactions will be investigated by in vitro reconstitution experiments. A comparison of internucleosome interactions in pre- and post-blastoderm Drosophila nuclei by this fractionation procedure will be made in an attempt to understand changes in nuclear morphology related to internucleosome packing interactions.